The diaryldiketopyrrolo[3,4-c]pyrrole (DPP) pigments are an important class of pigments on the worldwide market due to their outstanding chroma and excellent heatfastness and weatherfasmess properties. The DPP pigments and their preparation are well-known in the art. In addition, DPP pigment solid solutions are also known in the art.
The present invention provides a new process for conditioning DPP pigments and the preparation of pigment solid solutions containing at least one DPP wherein the DPP is dissolved in the form of a salt in dimethylsulfoxide and then precipitated.
Known processes for conditioning the DPP pigments include various milling methods, such as wet milling, or milling in an alcohol in the presence of a base, and recrystallization in organic solvents. Known processes for preparing solid solutions containing a DPP include recrystallization in organic solvents, acid or alkaline precipitation and intensive milling of a mixture containing the corresponding pigment components.
The present process is distinguished from known processes in that it relates to precipitating the DPP pigment or solid solution from aqueous, basic dimethylsulfoxide (DMSO) and is based on the discovery that aqueous, basic DMSO is capable of dissolving DPP pigment salts in high concentrations at temperatures below about 80.degree. C. Since
DMSO is readily available and is one of the least toxic polar organic solvents, the present process is a simple, economical and particularly environmentally friendly process. The present process is additionally advantageous over the known processes because highly transparent, semitransparent and/or opaque pigments having outstanding performance properties are prepared simply by varying process parameters.